.\"
.\" Copyright (c) 2009 Lawrence Stewart <lstewart@FreeBSD.org>
.\" Copyright (c) 2011 The FreeBSD Foundation
.\" All rights reserved.
.\"
.\" Portions of this documentation were written at the Centre for Advanced
.\" Internet Architectures, Swinburne University of Technology, Melbourne,
.\" Australia by Lawrence Stewart under sponsorship from the FreeBSD Foundation.
.\"
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.\" modification, are permitted provided that the following conditions
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.Dd February 4, 2023
.Dt CC_NEWRENO 4
.Os
.Sh NAME
.Nm cc_newreno
.Nd NewReno Congestion Control Algorithm
.Sh SYNOPSIS
.In netinet/cc/cc_newreno.h
.Sh DESCRIPTION
Details about the algorithm can be found in RFC5681.
.Sh Socket Options
The
.Nm
module supports a number of socket options under TCP_CCALGOOPT (refer to
.Xr tcp 4
and
.Xr mod_cc 9 for details)
which can
be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 .
The
.Nm
socket options use this structure defined in
<sys/netinet/cc/cc_newreno.h>:
.Bd -literal
struct cc_newreno_opts {
	int name;
	uint32_t val;
}
.Ed
.Bl -tag -width ".Va CC_NEWRENO_BETA_ECN"
.It Va CC_NEWRENO_BETA
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to a congestion signal per: cwnd = (cwnd *
CC_NEWRENO_BETA) / 100.
Default is 50.
.It Va CC_NEWRENO_BETA_ECN
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to an ECN congestion signal when
.Va net.inet.tcp.cc.abe=1
per: cwnd = (cwnd * CC_NEWRENO_BETA_ECN) / 100.
Default is 80.
.Pp
Note that currently the only way to enable
hystart++ is to enable it via socket option.
When enabling it a value of 1 will enable precise internet-draft (version 4) behavior
(subject to any MIB variable settings), other setting (2 and 3) are experimental.
.El
.Pp
Note that hystart++ requires the TCP stack be able to call to the congestion
controller with both the
.Va newround
function as well as the
.Va rttsample
function.
Currently the only TCP stacks that provide this feedback to the
congestion controller is rack.
.Sh MIB Variables
The algorithm exposes these variables in the
.Va net.inet.tcp.cc.newreno
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va beta_ecn"
.It Va beta
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to a congestion signal per: cwnd = (cwnd *
beta) / 100.
Default is 50.
.It Va beta_ecn
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to an ECN congestion signal when
.Va net.inet.tcp.cc.abe=1
per: cwnd = (cwnd * beta_ecn) / 100.
Default is 80.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_chd 4 ,
.Xr cc_cubic 4 ,
.Xr cc_dctcp 4 ,
.Xr cc_hd 4 ,
.Xr cc_htcp 4 ,
.Xr cc_vegas 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr mod_cc 9
.Rs
.%A "Mark Allman"
.%A "Vern Paxson"
.%A "Ethan Blanton"
.%T "TCP Congestion Control"
.%O "RFC 5681"
.Re
.Rs
.%A "Naeem Khademi"
.%A "Michael Welzl"
.%A "Grenville Armitage"
.%A "Gorry Fairhurst"
.%T "TCP Alternative Backoff with ECN (ABE)"
.%O "RFC 8511"
.Re
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh HISTORY
The
.Nm
congestion control algorithm first appeared in its modular form in
.Fx 9.0 .
.Pp
This was the default congestion control algorithm in FreeBSD before
version
.Fx 14.0 ,
after which
.Xr cc_cubic 4
replaced it.
.Pp
The module was first released in 2007 by James Healy and Lawrence Stewart whilst
working on the NewTCP research project at Swinburne University of Technology's
Centre for Advanced Internet Architectures, Melbourne, Australia, which was made
possible in part by a grant from the Cisco University Research Program Fund at
Community Foundation Silicon Valley.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
congestion control module was written by
.An James Healy Aq Mt jimmy@deefa.com ,
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org
and
.An David Hayes Aq Mt david.hayes@ieee.org .
.Pp
Support for TCP ABE was added by
.An Tom Jones Aq Mt tj@enoti.me .
.Pp
This manual page was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
